1. Field of the Invention
The present invention relates to a hetero-junction bipolar transistor, more particularly to a high output hetero-junction bipolar transistor having a multi-finger structure.
2. Description of the Related Art
A hetero-junction bipolar transistor is capable of achieving a high current gain, .beta., and is therefore used in high-frequency, high-output devices. Hetero-junction bipolar transistors using an AlGaAs/GaAs material have been commercialized.
In a hetero-junction bipolar transistor, in order to prevent the collector current from increasing due to heat generation and to prevent current concentration in a particular transistor in a multi-finger type high-output transistor set including a plurality of transistors, a GaAs ballast resistor layer having a relatively high resistance is located between an emitter AlGaAs layer and an emitter electrode to decrease the voltage and prevent a steep increase from occurring in the collector current.
FIG. 3 is an energy band diagram of an AlGaAs/GaAs hetero-junction bipolar transistor of the prior art. In FIG. 3, an n.sup.- -GaAs ballast resistor layer is provided between an n-AlGaAs emitter layer and an emitter electrode (not shown).
In the prior art hetero-junction bipolar transistor based on AlGaAs/GaAs, there has been a problem that the ballast resistance (resistance between the emitter layer and the emitter electrode) cannot be exactly controlled even when the concentration of impurities in the n.sup.- -GaAs ballast resistor layer is exactly controlled.
According to the present inventors' knowledge, it is considered that the problem of the prior art described above occurs as the pinning level of the band changes due to the effect of impurities trapped in the AlGaAs layer in the hetero-junction between the ballast resistor layer and the adjacent AlGaAs layer, thus causing the relative position of the lower end of the conduction band, with respect to the Fermi level E.sub.F, to change, depending on the quantity of impurities trapped.
That is, because the ballast resistor layer and the adjacent AlGaAs layer form a hetero-junction, a notch is formed at the lower end of the conduction band at the hetero-junction interface, as shown in FIG. 3, and the notch contributes to the increase in resistance. Thus it is supposed that, as the relative position of the lower end of the conduction band with respect to the Fermi level E.sub.F changes, the resistance due to the notch changes thereby making it impossible to exactly control the total ballast resistance even when the resistance of the ballast resistor layer is exactly controlled through precise control of the concentration of impurities in the ballast resistor layer and of the thickness of the ballast resistor layer.
In order to overcome this problem, the present inventors have studied a process of forming the ballast resistor layer from the same AlGaAs layer as the emitter layer. However, the AlGaAs layer is likely to trap impurities because of the Al component as described above. Such a constitution is also subject to a deterioration in current stability characteristics, that is, current changes significantly with time when a specified amount of current is supplied, supposedly due to the quality of the hetero-junction interface between the AlGaAs emitter layer and the GaAs base layer and the quality of the AlGaAs layer itself, thus resulting in deteriorating reliability of the device.